We are planning on building new home in Brisbane and speaking with builders

We thought that rain water tank will be great as we have a 1030 m2 land, the builder is costing it around 5000$ total to install and connect to toilet and laundry .

I did some math and looked at my current water bill and it appears we are paying only 3$ per 1000L. So for 5000l tank, we would be saving 15$.

Majority of the water bill is supply and sewage treatment charge, which i believe we would still need to pay regardless of whether I use 1 L of water or 5000 L - is that right?

If so, then probably for a year I would be saving only around 100$ roughly , so 5000k does not seem to be worth it. Am I missing something? It seems it's worth investing in Solar and it could be better returns.

Dunno about Brissy … how consistent is the rainfall?In Sydney it can be dry for a looong time, and while of course you can get water from the mains for anything important, stuff like car washing or watering the garden isn't legal in times of water-restriction. But if you've got a tank, you can use that water for your garden.